In recent years, there has been a marked increase in the use of high technology engines in mass-produced automobiles. Features such as single overhead camshafts (SOHC) or double overhead camshafts (DOHC), and aluminium alloy engines, are now supplied standard on most cars. Modern manufacturing techniques have also enabled automobile engine designers to design to much closer manufacturing tolerances than had previously been attainable, and this has led to much improved mechanical efficiencies within the engine, and much lighter engine assemblies.
Unfortunately, engine repair and rebuilding techniques have not kept pace with this new technology. Procedures and tools which worked adequately on older engines having tolerances in the order to .+-.0.25 mm (.+-.0.010 inches) are not normally capable of producing tolerances in the order of .+-.0.05 mm (.+-.0.002 inches) as required by modern SOHC and DOHC engines.
These changes have been particularly noticeable in the design and repair of cylinder heads. In older engines, the camshaft that operates the inlet and exhaust valves is usually located deep within the motor, adjacent to the crankshaft, and the valves are actuated by long pushrods operating off the cams of the camshaft, or by hydraulic means. The cylinder head of these motors merely acts as a seal for the cylinders, and a mounting base on which to mount the valves and valve rockers, and any free-play between the valves, valve rockers and pushrods can normally be eliminated by using shims, or adjustable screws. As mentioned above, a manufacturing tolerance of the order of .+-.0.25 mm is sufficient for the flat mounting surface of the cylinder head where it is mounted onto the engine block. A good gasket can normally ensure that the cylinder head seals the cylinders properly, even if the head has a small degree of twist (or warp), or is bowed, within this .+-.0.25 mm tolerance.
The procedures and tools commonly used to straighten an aluminium (alloy) cylinder head of this type are somewhat simple. First, a number of holes are drilled and tapped into a piece of flat steel plate to suit the mounting bolt holes of the particular cylinder head to be straightened. The head is measured while it is cold and the location and amount of distortion are calculated. The head is then placed on the steel plate and shims are placed between the head and steel plate at the contact points. Bolts are passed through the mounting holes in the head, screwed into the steel plate, and tightened to a torque that has been estimated empirically to give the required head straightness. The whole assembly is put in an oven to anneal for a period of 2-4 hours. After this time, it is removed from the oven, the bolts are loosened, and the head is checked to ascertain whether it has straightened sufficiently so as to be usable. If the head is still not straight enough, the whole procedure is repeated until the desired degree of straightness is achieved. Hence, this trial-and-error procedure is greatly dependent upon the skill and knowledge of the tradesman doing the job. However, with the wide tolerance allowable on the flat head mounting surface of most old cylinder heads, the head is usually "straight" after one annealing process.
With overhead cam (OHC) engines on the other hand, the camshaft is journalled in a series of bearings on the aluminium (alloy) cylinder head, and the valves are operated directly by cams on the camshaft. Little or no adjustment is available with this arrangement, so manufacturing tolerances are much stricter than the earlier arrangements described above. The most critical manufacturing tolerance in OHC arrangements is in the alignment of the support bearings for the camshaft. The (four or five) bearings supporting the camshaft must all be aligned so that their rotational centrelines are concentric to within .+-.0.05 mm as stated earlier. With this tolerance in mind, it can be readily appreciated that the abovementioned prior art procedure for straightening cylinder heads is not suited to straightening OHC heads as it does not allow for accurate control or measurement of the amount of straightening. Indeed, an OHC head often requires several heat treatments to achieve the desired degree of straightness. This is both time consuming and expensive, and in most cases, it is more economical to replace a deformed cylinder head rather than attempt to straighten it.
Further, it is usually necessary to drill and tap new holes in the steel plate for different makes of cylinder heads.